1. Field of the Invention
The present invention relates to a starter for starting an engine.
2. Description of the Prior Art
FIG. 9 is a sectional view showing an example of a conventional starter disclosed in Japanese Published Unexamined Patent Application No. 10-159693-1998.
In FIG. 9, 1A is an output shaft. An electromagnetic switch 2A, an over-running clutch 30A provided with a pinion 30P which meshes with a ring gear 50A, a plunger 40A comprising an inner plunger 4A and an outer plunger 4B, etc. are arranged coaxially on this output shaft 1A. A starter with this structure is generally called a coaxial type starter. 12A is an armature of a DC electric motor and 16A is a shaft (a motor shaft). 18A is a reduction mechanism which reduces the rotational force of the shaft 16A and transmits it to the output shaft 1A.
8A is a contact shaft supported by an inner gear member 17A of the reduction mechanism 18A almost parallel with the plunger 40A through a supporting hole 17m.
100 is a bracket and 800 is a shift plate which connects the outer plunger 4B with the contact shaft 8A.
The upper portion from the central axis in FIG. 9 shows the state of a starter not in operation and the lower portion shows the state wherein the starter is in operation with an electromagnetic switch turned ON and the pinion meshed with the ring gear.
Next, the operation of the starter is explained. The operation will be explained referring also to FIG. 10, which is a partially enlarged view of FIG. 9.
First, when an ignition switch is turned ON and current flows to an exciting coil 2B of the electromagnetic switch 2A, the outer plunger 4B is attracted by the exciting core 2C of the electromagnetic switch 2A. This conventional starter has such a structure that the outer plunger 4B is directly connected with the contact shaft 8A via the shift plate 800. When the outer plate 4B is attracted by the exciting coil 2B, the contact shaft 8A is also moved simultaneously. Between the outer plunger 4B and the inner plunger 4A, there is a coil spring 401 mounted via a spring bracket 400. The inner plunger 4A is kept in the stationary state because the coil spring 401 bends at the initial stage even when the outer plunger 4B is attracted and begins to move. In front of the inner plunger 4A, an inner clutch 30B is mounted via a shifter member 402 and as long as the inner plunger 4A is kept in the stationary state, the inner clutch 30B is also kept in the stationary state. After a short interval when the plunger 4B is attracted and begins to move, a movable contact 80A mounted on the contact shaft 8A comes into contact with a stationary contact 80B mounted in the contact chamber ZA. When the movable contact 80A is brought into contact with the stationary contact 80B, electric power is supplied from an external power source via a contact bolt 11A and an armature 12A begins to turn. When the output shaft 1A begins to turn by way of the reduction mechanism 18A, the pinion 30P is caused to move toward the ring gear 50A by a thrust generated in a helical spline portion 1B, and the threads and the thread grooves of the pinion 30P and the ring gear 50A agree and mesh. Thereafter, when the engine starts, the output shaft 1A and the pinion 30P are separated by the action of the overrunning clutch 30A and the pinion runs idle. When the power supply to the exciting coil 2B is stopped, the pinion 30P is disengaged from the ring gear 50A by return springs 403, 404.
However, in the case of a conventional starter disclosed in Japanese Published Unexamined Patent Application No. 10-159693-1998, the outer plunger 4B is directly connected with the contact shaft 8A by the shift plate 800. The contact shaft 8A also moves simultaneously with the attraction and movement of the outer plunger 4B and therefore, the movable contact 80A immediately comes into contact with the stationary contact 80B and the armature 12A begins to rotate before the end surface 30Pe of the pinion 30P comes into contact with the end surface 50Ae of the ring gear 50A.
That is, according to the above conventional starter, the pinion 30P meshes with the ring gear 50A by rotating the pinion 30P by driving the armature 12. However, in the above starter, the thrust generated in the helical spline portion 1B is insufficient to press the pinion 30P toward the ring gear 50A and the coil spring 401 is also incapable of pressing the pinion 30P toward the ring gear 50A. Therefore, when meshing with the ring gear 50A, the pinion 30P is often repelled by the ring gear 50A and tries again to mesh with it.
Accordingly, the pinion 30P does not mesh with the ring gear 50A smoothly and reliability when the pinion 30P meshes inadequately with the ring gear 50A. In addition, the gears are worn away and the life span of the gears is shortened.
In the case of a starter with a structure wherein the contact shaft moves by the same amount as the outer plunger as in the above starter, it is required to secure a moving space for a plate (an engaging ring, etc.) to hold the coil spring 9S for pressing the contact shaft and the movable contact in the contact chamber and, as a result, the contact chamber inevitably becomes large.